This invention relates to a process for synthesizing molecular sieves. More specifically, the process involves adding nutrients (sources) of the framework elements, e.g., aluminum and silicon to a slurry of seed crystals of the molecular sieve. The rate of addition of the nutrients is controlled so that it is substantially the same as the crystal growth rate.
Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art, and now comprise over 150 species of both naturally occurring and synthetic zeolites. In general, the crystalline zeolites are formed from corner-sharing AlO2 and SiO2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions are known which are not zeolitic but which exhibit the ion exchange and/or adsorption characteristics of the zeolites. These include: 1) a pure silica polymorph, silicalite, having a neutral framework containing neither cations nor cation sites as disclosed in U.S. Pat. No. 4,061,724; 2) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 3) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871 and 4) titanium substituted aluminophosphates as disclosed in U.S. Pat. No. 4,500,651.
Molecular sieves are usually hydrothermally synthesized from a reaction mixture in a batch reactor. In this type of process, all ingredients are added to a reactor thereby forming a gel. The gel is next stirred and heated for a sufficient time to crystallize the zeolite. The drawbacks to a conventional process include limitations on the control of size and morphology of crystals, limitations on the solids content, generation of waste products which can""t be recycled and large capital investments. Accordingly, the industry is continuously conducting research to improve the manufacture of molecular sieves.
For example, U.S. Pat. No. 4,314,979 discloses a continuous process for preparing zeolite A. The process involves mixing solutions containing aluminum and silicon and flowing the mixture to a crystallization reactor to crystallize zeolite A. U.S. Pat. No. 5,389,358 discloses a process for synthesizing zeolites by first nucleating crystals and then adding solutions which contain the reactants followed by aging in order to crystallize the zeolite. Finally, U.S. Pat. No. 3,425,800 describes a continuous process for synthesizing zeolite A or X in which aqueous solutions of the reactants are mixed to form a gel, the gel is heated and then supplied to a stratified crystallization zone where the crystals form.
C. S. Cundy et al., in Zeolites, Vol. 15, 353-372 (1995), discloses a process for synthesizing the zeolite ZSM-5. The process involves filling a reactor with a slurry of seed crystals in a suitable liquid. To this mixture there are continuously added sources of aluminum and silicon with intermittent removal of product such that the reactor is filled to a constant level. In a second article by the same authors (Zeolites, Vol. 15, 400-407 (1995) it is disclosed that when aluminum and silicon are added at a faster rate than crystal growth, a high nucleation rate is observed.
In contrast to these references, applicants have developed a process in which seed crystals are grown to a desired size. The process involves adding to a slurry of seed crystals nutrients which are sources of the framework elements, e.g., Al, Si, of the molecular sieve. Nutrients are added at a rate which essentially equals the crystal growth rate, such that no gel is formed and there is substantially no nucleation of new crystals. The nutrients can provide framework elements different from the framework elements of the seed crystals but which produce the same framework structure as the seed crystals. Mixing of the slurry is controlled such that either single crystals or agglomerates are obtained. The addition of nutrients is carried out until the desired crystal size or particle size is obtained at which point the molecular sieve is separated from the liquid by conventional means.
As stated, this invention relates to a process for the synthesis of molecular sieves. Accordingly, one embodiment of the invention is a process for synthesizing a molecular sieve having a three-dimensional microporous framework structure and a framework composition represented by an empirical formula of
(AlxSi1xe2x88x92x)O2
where Al and Si are framework elements present as tetrahedral oxide units, x has a value from 0 to about 0.5; the process comprising providing a slurry of seed crystals at reaction conditions; adding to the slurry nutrients, to provide framework elements of the seed crystals thereby growing the seed crystals; carrying out the addition at a rate that essentially equals the crystal growth rate and for a time sufficient to produce the molecular sieve.
Another embodiment of the invention is to use the process described in the previous paragraph to prepare a molecular sieve represented by an empirical formula of:
(ElwAlxPyxe2x80x2Siz)O2
where El, Al, P and Si are framework elements present as tetrahedral oxide units, xe2x80x9cwxe2x80x9d is the mole fraction of El and has a value from zero to about 0.5, xe2x80x9cxxe2x80x9d is the mole fraction of Al and has a value from 0 to about 0.5, yxe2x80x2 is the mole fraction of P and has a value from greater than 0 to about 0.5, and xe2x80x9czxe2x80x9d is the mole fraction of Si and has a value from 0 to about 0.98, w+x+yxe2x80x2+z=1.
Yet another embodiment of the invention is a process for synthesizing a microporous molecular sieve having a three dimensional structure comprising a core molecular sieve and an outer molecular sieve, both molecular sieves having the same framework structure, the core molecular sieve having a composition represented by an empirical formula of:
(AlxSi1xe2x88x92x)O2
where Al and Si are framework elements, present as tetrahedral oxide units, and x has a value from 0 to about 0.5; the process comprising providing a slurry of crystals of the core molecular sieve at reaction conditions; adding to the slurry nutrients to provide framework elements, thereby growing an outer molecular sieve over the crystals, the outer molecular sieve having the same framework structure as the core molecular sieve but the core and outer molecular sieve differing by at least one framework element, the outer molecular sieve having a composition represented by the empirical formula:
(ElwAlxPySiz)O2
where El, Al, P and Si are framework elements present as tetrahedral oxide units, xe2x80x9cwxe2x80x9d is the mole fraction of El and has a value from zero to about 0.5, xe2x80x9cxxe2x80x9d is the mole fraction of Al and has a value of 0 to about 0.5, xe2x80x9cyxe2x80x9d is the mole fraction of P and has a value from 0 to about 0.5, and xe2x80x9czxe2x80x9d is the mole fraction of Si and has a value from 0 to about 1, w+x+y+z=1 and xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d are not simultaneously zero; carrying out the addition at a rate that essentially equals the growth rate of the outer molecular sieve and for a time sufficient to produce the molecular sieve.
A further embodiment of the invention is a process for synthesizing a microporous molecular sieve having a three dimensional structure comprising a core molecular sieve and an outer molecular sieve, both molecular sieves having the same framework structure, the core molecular sieve having a composition represented by an empirical formula of:
(ElwAlxPyxe2x80x2Siz)O2
where El, Al, P and Si are framework elements present as tetrahedral oxide units, xe2x80x9cwxe2x80x9d is the mole fraction of El and has a value from zero to about 0.5, xe2x80x9cxxe2x80x9d is the mole fraction of Al and has a value from 0 to about 0.5, yxe2x80x2 is the mole fraction of P and has a value from greater than 0 to about 0.5, and xe2x80x9czxe2x80x9d is the mole fraction of Si and has a value from 0 to about 0.98, w+x+yxe2x80x2+z=1, the process comprising providing a slurry of crystals of the core molecular sieve at reaction conditions; adding to the slurry nutrients to provide framework elements thereby growing an outer molecular sieve over the crystals, the outer molecular sieve having the same framework structure as the core molecular sieve but the core and outer molecular sieve differing by at least a one framework element, the outer molecular sieve having a composition represented by the empirical formula:
(ElwAlxPySiz)O2
where El, Al, P and Si are framework elements present as tetrahedral oxide units, xe2x80x9cwxe2x80x9d is the mole fraction of El and has a value from zero to about 0.5, xe2x80x9cxxe2x80x9d is the mole fraction of Al and has a value from 0 to about 0.5, xe2x80x9cyxe2x80x9d is the mole fraction of P and has a value from 0 to about 0.5, and xe2x80x9czxe2x80x9d is the mole fraction of Si and has a value from 0 to about 1, w+x+y+z=1 and xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d are not simultaneously zero; carrying out the addition at a rate that essentially equals the growth rate of the outer molecular sieve and for a time sufficient to produce the molecular sieve.
These and other objects, embodiments and details of this invention will become apparent after a detailed description of the invention.